1. Field of the Invention
The present invention relates to a method for manufacturing a Group III nitride semiconductor light-emitting element having a light-emitting diode (LED) structure, a Group III nitride semiconductor light-emitting element, a lamp, and a reticle that is used for the manufacturing of the Group III nitride semiconductor light-emitting element.
Priority is claimed on Japanese Patent Application No. 2012-038612, filed on Feb. 24, 2012, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, as a semiconductor material for a light-emitting element that emits light with a short wavelength, a Group III nitride semiconductor has attracted attention. The Group III nitride semiconductor is expressed by a general formula AlxGayInzN (0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1), and is formed on a substrate formed from various oxides such as a sapphire single crystal or Group III-V compound by a metalorganic chemical vapor deposition (MOCVD) method, a molecular beam epitaxy (MBE) method, or the like.
In a general light-emitting element using the Group III nitride semiconductor, an n-type semiconductor layer formed from the Group III nitride semiconductor, a light-emitting layer, and a p-type semiconductor layer are laminated on a substrate formed from a sapphire single crystal in this order. The sapphire substrate is an insulating body, and thus, generally, an element structure has a structure in which a positive electrode formed on the p-type semiconductor layer and a negative electrode formed on the n-type semiconductor layer are present in the same plane. In this Group III nitride semiconductor light-emitting element, two kinds of element are present including a face-up type in which a transparent electrode is used for the positive electrode, and light is extracted from the p-type semiconductor side, and a flip-chip type in which a highly reflective film such as Ag is used for the positive electrode, and light is extracted from the sapphire substrate side.
As an output index of this light-emitting element, external quantum efficiency is used. When the external quantum efficiency is high, it can be said that the light-emitting element has a high output. The external quantum efficiency is a value obtained by multiplying internal quantum efficiency and light extraction efficiency. The internal quantum efficiency represents a conversion ratio of energy of a current, which is injected to the element, into light at the light-emitting layer. The light extraction efficiency represents a ratio of light, which may be extracted to the outside of the light-emitting element, in the light that is generated at the light-emitting layer. Accordingly, it is necessary to improve the light extraction efficiency so as to improve the external quantum efficiency.
As a method of improving the light extraction efficiency, two methods may be exemplified. One is a method of reducing light absorption due to an electrode or the like that is formed on a light extraction surface. The other is a method of reducing light confinement inside the light-emitting element, which occurs due to a difference in a refractive index between the light-emitting element and an external medium.
As a method of reducing the light confinement of the light at the inside of the light-emitting element, a technology in which concavity and convexity are formed in a light extraction surface of the light-emitting element may be exemplified (for example, refer to Japanese Patent No. 2836687).
However, in the light-emitting element in which the concavity and convexity are formed in the light extraction surface by mechanical processing or chemical processing, a load that is caused by the processing in the light extraction surface is given to the semiconductor layer, and thus damage is caused to the light-emitting layer. In addition, in the light-emitting element in which the semiconductor layer is grown under conditions in which the concavity and convexity are formed in the light extraction surface, the crystallinity of the semiconductor layer deteriorates, and thus the light-emitting layer contains defects. Therefore, in the case of forming the concavity and convexity in the light extraction surface, the light extraction efficiency is improved, but the internal quantum efficiency decreases. As a result, there is a problem in that it is difficult to increase light emission intensity.
Therefore, a method is suggested in which the concavity and convexity are formed in a surface of a substrate formed from sapphire instead of forming the concavity and convexity in the light extraction surface, and the Group III nitride semiconductor layer is grown on the concavity and convexity (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2002-280611). Furthermore, a method is suggested in which a GaN crystal is made to grow on a substrate having a curved convex portion that is formed as a concavo-convex shape of the substrate (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2005-129896). In addition, a method is also suggested in which a buffer layer is formed on a light-transmitting substrate having a concavo-convex shape according to a sputtering method using a sputtering device (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2007-273659). In addition, a semiconductor light-emitting element is suggested in which as a concavo-convex arrangement pattern on a substrate, convex portions are periodically disposed in the plane of the substrate with approximately the same interval in each of two axial directions intersecting each other in a predetermined angle, and the semiconductor layer is formed on the substrate (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2005-101566).
Particularly, according to each of the methods described in Japanese Unexamined Patent Application, First Publication Nos. 2002-280611, 2005-129896, 2007-273659, and 2005-101566, since the interface between the sapphire substrate and the Group III nitride semiconductor layer has a concavo-convex shape, the light confinement inside the light-emitting element may be reduced by scattered reflection of light on the interface due to a difference in a refractive index between the sapphire substrate and the Group III nitride semiconductor layer, and thus the light extraction efficiency may be improved. In addition, it is possible to reduce crystal defects by using the growth of the crystal in a lateral direction due to the concavity and convexity formed in the surface of the sapphire substrate, and it is possible to improve the internal quantum efficiency.